NAME

masq - Shorewall Masquerade/SNAT definition file

SYNOPSIS

/etc/shorewall/masq

DESCRIPTION

Use this file to define dynamic NAT (Masquerading) and to define Source
NAT (SNAT).
Warning
The entries in this file are order-sensitive. The first entry that
matches a particular connection will be the one that is used.
Warning
If you have more than one ISP link, adding entries to this file
will not force connections to go out through a particular link. You
must use entries in shorewall-route_rules[1](5) or PREROUTING
entries in shorewall-tcrules[2](5) to do that.
The columns in the file are as follows.
INTERFACE -
{[+]interfacelist[:[digit]][:[address[,address]...[exclusion]]|COMMENT}
Outgoing interfacelist. This may be a comma-separated list of
interface names. This is usually your internet interface. If
ADD_SNAT_ALIASES=Yes in shorewall.conf[3](5), you may add ":" and a
digit to indicate that you want the alias added with that name
(e.g., eth0:0). This will allow the alias to be displayed with
ifconfig. Thatistheonlyuseforthealiasname;itmaynotappearinanyotherplaceinyourShorewallconfiguration.
Each interface must match an entry in shorewall-interfaces[4](5).
Shorewall allows loose matches to wildcard entries in
shorewall-interfaces[4](5). For example, ppp0 in this file will
match a shorewall-interfaces[4](5) entry that defines ppp+.
Where morethatoneinternetprovidershareasingleinterface[5],
the provider is specified by including the provider name or number
in parentheses:
eth0(Avvanta)
In that case, you will want to specify the interfaces's address for
that provider in the ADDRESS column.
The interface may be qualified by adding the character ":" followed
by a comma-separated list of destination host or subnet addresses
to indicate that you only want to change the source IP address for
packets being sent to those particular destinations. Exclusion is
allowed (see shorewall-exclusion[6](5)).
If you wish to inhibit the action of ADD_SNAT_ALIASES for this
entry then include the ":" but omit the digit:
eth0(Avvanta):
eth2::192.0.2.32/27
Normally Masq/SNAT rules are evaluated after those for one-to-one
NAT (defined in shorewall-nat[7](5)). If you want the rule to be
applied before one-to-one NAT rules, prefix the interface name with
"+":
+eth0
+eth0:192.0.2.32/27
+eth0:2
This feature should only be required if you need to insert rules in
this file that preempt entries in shorewall-nat[7](5).
Comments may be attached to Netfilter rules generated from entries
in this file through the use of COMMENT lines. These lines begin
with the word COMMENT; the remainder of the line is treated as a
comment which is attached to subsequent rules until another COMMENT
line is found or until the end of the file is reached. To stop
adding comments to rules, use a line with only the word COMMENT.
SOURCE (Formerly called SUBNET) -
{interface[:exclusion]|address[,address][exclusion]}
Set of hosts that you wish to masquerade. You can specify this as
an address (net or host) or as an interface (use of an interface is
deprecated). If you give the name of an interface, the interface
must be up before you start the firewall and the Shorewall rules
compiler will warn you of that fact. (Shorewall will use your main
routing table to determine the appropriate addresses to
masquerade).
In order to exclude a address of the specified SOURCE, you may
append an exclusion ("!" and a comma-separated list of IP addresses
(host or net) that you wish to exclude (see
shorewall-exclusion[6](5))). Note that a colon (":") must appear
between an interface name and the exclusion;
Example: eth1:!192.168.1.4,192.168.32.0/27
In that example traffic from eth1 would be masqueraded unless it
came from 192.168.1.4 or 196.168.32.0/27
ADDRESS (Optional) -
[-|NONAT|[address-or-address-range[,address-or-address-range]...][:lowport-highport][:random][:persistent]|detect|random]
If you specify an address here, SNAT will be used and this will be
the source address. If ADD_SNAT_ALIASES is set to Yes or yes in
shorewall.conf[3](5) then Shorewall will automatically add this
address to the INTERFACE named in the first column.
You may also specify a range of up to 256 IP addresses if you want
the SNAT address to be assigned from that range in a round-robin
fashion by connection. The range is specified by
first.ip.in.range-last.ip.in.range. You may follow the port range
with :random in which case assignment of ports from the list will
be random. random may also be specified by itself in this column
in which case random local port assignments are made for the
outgoing connections.
Example: 206.124.146.177-206.124.146.180
You may follow the port range (or :random) with :persistent. This
is only useful when an address range is specified and causes a
client to be given the same source/destination IP pair. This
feature replaces the SAME modifier which was removed from Shorewall
in version 4.4.0. Unlike random, persistent may not be used by
itself.
You may also use the special value "detect" which causes Shorewall
to determine the IP addresses configured on the interface named in
the INTERFACES column and substitute them in this column.
Finally, you may also specify a comma-separated list of ranges
and/or addresses in this column.
This column may not contain DNS Names.
Normally, Netfilter will attempt to retain the source port number.
You may cause netfilter to remap the source port by following an
address or range (if any) by ":" and a port range with the format
lowport-highport. If this is done, you must specify "tcp" or "udp"
in the PROTO column.
Examples:
192.0.2.4:5000-6000
:4000-5000
If you simply place NONAT in this column, no rewriting of the
source IP address or port number will be performed. This is useful
if you want particular traffic to be exempt from the entries that
follow in the file.
If you want to leave this column empty but you need to specify the
next column then place a hyphen ("-") here.
PROTO (Optional) - {-|[!]protocol-name|[!]protocol-number}
If you wish to restrict this entry to a particular protocol then
enter the protocol name (from protocols(5)) or number here.
PORT(S) (Optional) - [[!]port-name-or-number[,port-name-or-number]...]
If the PROTO column specifies TCP (6), UDP (17), DCCP (33), SCTP
(132) or UDPLITE (136) then you may list one or more port numbers
(or names from services(5)) separated by commas or you may list a
single port range (lowport:highport).
Where a comma-separated list is given, your kernel and iptables
must have multiport match support and a maximum of 15 ports may be
listed.
IPSEC (Optional) - [option[,option]...]
If you specify a value other than "-" in this column, you must be
running kernel 2.6 and your kernel and iptables must include policy
match support.
Comma-separated list of options from the following. Only packets
that will be encrypted via an SA that matches these options will
have their source address changed.
reqid=number
where number is specified using setkey(8) using the
'unique:number option for the SPD level.
spi=<number>
where number is the SPI of the SA used to encrypt/decrypt
packets.
proto=ah|esp|ipcomp
IPSEC Encapsulation Protocol
mss=number
sets the MSS field in TCP packets
mode=transport|tunnel
IPSEC mode
tunnel-src=address[/mask]
only available with mode=tunnel
tunnel-dst=address[/mask]
only available with mode=tunnel
strict
Means that packets must match all rules.
next
Separates rules; can only be used with strict
MARK - [!]value[/mask][:C]
Defines a test on the existing packet or connection mark. The rule
will match only if the test returns true.
If you don't want to define a test but need to specify anything in
the following columns, place a "-" in this field.
!
Inverts the test (not equal)
value
Value of the packet or connection mark.
mask
A mask to be applied to the mark before testing.
:C
Designates a connection mark. If omitted, the packet mark's
value is tested.
USER/GROUP (Optional) -
[!][user-name-or-number][:group-name-or-number][+program-name]
Only locally-generated connections will match if this column is
non-empty.
When this column is non-empty, the rule matches only if the program
generating the output is running under the effective user and/or
group specified (or is NOT running under that id if "!" is given).
Examples:
joe
program must be run by joe
:kids
program must be run by a member of the 'kids' group
!:kids
program must not be run by a member of the 'kids' group
+upnpd
#program named upnpd
Important
The ability to specify a program name was removed from
Netfilter in kernel version 2.6.14.

EXAMPLES

Example 1:
You have a simple masquerading setup where eth0 connects to a DSL
or cable modem and eth1 connects to your local network with subnet
192.168.0.0/24.
Your entry in the file can be either:
#INTERFACE SOURCE
eth0 eth1
or
#INTERFACE SOURCE
eth0 192.168.0.0/24
Example 2:
You add a router to your local network to connect subnet
192.168.1.0/24 which you also want to masquerade. You then add a
second entry for eth0 to this file:
#INTERFACE SOURCE
eth0 192.168.1.0/24
Example 3:
You have an IPSEC tunnel through ipsec0 and you want to masquerade
packets coming from 192.168.1.0/24 but only if these packets are
destined for hosts in 10.1.1.0/24:
#INTERFACE SOURCE
ipsec0:10.1.1.0/24 196.168.1.0/24
Example 4:
You want all outgoing traffic from 192.168.1.0/24 through eth0 to
use source address 206.124.146.176 which is NOT the primary address
of eth0. You want 206.124.146.176 to be added to eth0 with name
eth0:0.
#INTERFACE SOURCE ADDRESS
eth0:0 192.168.1.0/24 206.124.146.176
Example 5 (using the deprecated form with an interface name in the
SOURCE column):
You want all outgoing SMTP traffic entering the firewall on eth1 to
be sent from eth0 with source IP address 206.124.146.177. You want
all other outgoing traffic from eth1 to be sent from eth0 with
source IP address 206.124.146.176.
#INTERFACE SOURCE ADDRESS PROTO PORT(S)
eth0 eth1 206.124.146.177 tcp smtp
eth0 eth1 206.124.146.176
Warning
The order of the above two rules is significant!